One of Tony Blair's closest political advisers became the first government official to be arrested in the "cash for peerages" inquiry when Scotland Yard officers detained her for six hours following a dawn raid at her home.

Ruth Turner, the prime minister's political liaison officer since 2005, was also the first person to be questioned on suspicion of perverting the course of justice.

The surprise development in the 10-month investigation provoked Mr Blair to issue an immediate statement supporting Ms Turner, who also "refuted absolutely" any wrongdoing.

Four officers arrived at her London home without warning at 6.30am yesterday and in a statement Scotland Yard, whose inquiry has been led by Assistant Commissioner John Yates, said the move came "in connection with alleged offences under the Honours (Prevention of Abuses) Act 1925 and also on suspicion of perverting the course of justice". She was later released on police bail.

Police refused to elaborate on the arrest, but it is known that detectives have gathered hundreds of documents and emails from Downing Street.

Acts that may constitute perverting the course of justice include withholding or destroying documents, or interfering with potential witnesses. Yesterday's arrest also raised the prospect that detectives may yet re-interview other members of the No 10 staff before sending their final reports to the Crown Prosecution Service.

No 10 remains adamant that it did not offer the prospect of honours to rich supporters who made undisclosed loans before the last election, the original charge in an SNP complaint to the police.

The day's drama coincided with the release in the Commons of a letter from Lord Goldsmith, the attorney general, setting out why he will not stand aside from advising the CPS on such a sensitive case - and why Lord Falconer, the lord chancellor, was wrong to suggest he would.

The admonishment was disclosed in a letter released by the Commons constitutional affairs committee and reignited concerns by Conservative MPs that Lord Goldsmith, a friend and political ally of the Mr Blair, ought to distance himself from the inquiry to prevent accusations that Labour could be seen intervening to prevent possible prosecution of donors and aides. Within an hour of Scotland Yard's disclosure that officers had arrested Ms Turner, Mr Blair issued a strong statement of "complete confidence" in his political secretary. Officials voiced private astonishment that the police had felt the need to arrest her before dawn.

In her own statement, Ms Turner added: "I have been completely open with the police throughout and will continue to cooperate with them fully. I absolutely refute any allegations of wrongdoing of any nature whatsoever."

No 10 insiders dismiss speculation that Ms Turner's boss, Jonathan Powell, the chief of staff, will be interviewed again, or that Mr Blair will now be seen a second time. But they were caught unaware by yesterday's development.

Some 90 people in all parties have been interviewed, but the Blair camp insists there is "not a strong correlation between the volume of noise and the evidence". Despite jitters and police-inspired reports that between two and six people could face charges, No 10 says there is "no smoking gun". Last night one official cited yesterday's arrest as proof that this is a normal police inquiry. "People originally thought it would be a stunt. It isn't. Then they said it would be a fix. it isn't. It's an absolutely standard investigation. The police arrest people, sometimes at six in the morning."

Angus MacNeil, the SNP MP whose complaint sparked the Met's inquiry, said: "Water is now lapping around Blair's neck. This investigation is now right inside the door of No 10." The Conservatives, which was the first party to take undeclared election loans via a legal loophole, made no comment, but the Liberal Democrats called for urgent reform of party funding.